1. Field of the Invention
The present invention relates to a piezoelectric resonator having temperature compensation and an improved electromechanical coupling coefficient, and to a method for manufacturing same.
2. Description of Prior Art
Piezoelectric resonators are being employed in electrical devices to an increasing extent. Piezoelectric resonators are, for example, suitable for being used in mobile telephones to filter out a frequency from a frequency spectrum. Apart from mobile telephones, all modules allowing wireless communication are generally typical applications of piezoelectric resonators. These modules are, for example, employed in laptop computers or PDAs to communicate wirelessly with a data network.
A piezoelectric resonator consists of at least two electrodes between which a piezoelectric layer is arranged. A special case of the piezoelectric resonator is the so-called BAW resonator (BAW=bulk acoustic wave) comprising high or low electrical impedance depending on a frequency of the alternating voltage applied. A BAW filter here functions like a switch which is closed when the frequency of the alternating voltage at the two electrodes is within a resonance frequency band and which is open when the frequency of the alternating voltage is not in the resonance frequency band.
The resonance frequency of the piezoelectric resonators and thus also of the bulk acoustic wave resonators depends on temperature. The temperature dependence of the resonance frequency is a decisive performance characteristic for piezoelectric resonators, because it determines in which range the resonance frequency of the piezoelectric resonator varies when operated. When operating the piezoelectric resonators, it must be kept in mind that the limits of the pass region of the piezoelectric resonator employed are selected such that the frequency to be filtered out is within the limits of the pass region over the entire temperature range. A reduction of the temperature dependence of the resonance frequency of the piezoelectric resonator allows manufacturing these filters with improved characteristics but also with an increased manufacturing yield. When testing the piezoelectric resonators only those elements where the resonance frequency is within limits predetermined by the manufacturer over the entire specified temperature range of the piezoelectric resonator are classified as being suitable for delivery.
This requirement to piezoelectric resonators may result in a deterioration in the pass band behavior or the manufacturing yield in production.
FIG. 7 shows a conventional piezoelectric resonator. Three conducting layers 3 and two insulating layers 5 are applied to a substrate 1 alternatingly. A first electrode 7a is arranged on the uppermost conducting layer 5. The first electrode 7a is covered by a first acoustic densification layer 9a onto which a piezoelectric layer 11 is deposited. The piezoelectric layer 11 in turn is covered by a second acoustic densification layer 9b on which there is the second electrode 7b. 
When applying an alternating voltage between the first electrode 7a and the second electrode 7b, a mechanical wave forms in the multi-layered setup. At a resonance frequency, there is a current in the electrodes 7a, 7b. The resonance frequency of the piezoelectric resonator shown here depends on a form and dimensions of the multi-layered setup, ambient temperature and the materials used in the multi-layered setup.
It is, however, of disadvantage in the conventional piezoelectric resonator shown in FIG. 7 that the value of the resonance frequency is highly dependent on temperature. This basically results from a temperature dependence of the influence of the piezoelectric layer 11 on the value of the resonance frequency of the piezoelectric resonator. The so-called resonance frequency temperature coefficient of the material of which the piezoelectric layer 11 is formed is a measure of the influencing of the value of the resonance frequency of the piezoelectric resonator.
The acoustic densification layers 9a, 9b are characterized by a high acoustic impedance and provide for mechanical waves only to propagate to a small extent from the piezoelectric layer to other layers of the multi-layered setup shown here, in particular the electrodes 7a, 7b. At the same time, the acoustic densification layers 9a, 9b are formed of a material having a low resistivity, which is why they are characterized by a high electrical conductivity.
A number of methods are feasible for reducing the temperature dependence of the resonance frequency of the piezoelectric resonators. However, the only method which may be executed easily is depositing an amorphous silicon dioxide layer onto the piezoelectric layer 11 such that the multi-layered setup arranged between the two electrodes 7a, 7b comprises, among other things, the piezoelectric layer 11 and the amorphous silicon dioxide layer.
The amorphous silicon dioxide has a positive resonance frequency temperature coefficient, whereas materials from which the piezoelectric layer 11 is formed, preferably comprise a negative resonance frequency temperature coefficient. In order to obtain a marked improvement in the temperature dependence of the resonance frequency, it is necessary to arrange the layer of the amorphous silicon dioxide, which is also referred to as compensation or temperature compensation layer, between one of the two electrodes 7a, 7b and the piezoelectric layer 11, and preferably between one of the two acoustic densification layers 9a, 9b and the piezoelectric layer 11. In this region between one of the two electrodes 7a, 7b and the piezoelectric layer 11, the mechanical waves occurring when operating the piezoelectric resonator comprise a high amplitude.
This method of arranging an amorphous silicon dioxide layer between one of the two electrodes 7a, 7b and the piezoelectric layer 11 is suitable for compensating the temperature dependence of the resonance frequency, it entails, however, a reduction in the value of the electromechanical coupling coefficient Keff, which results in the pass region of the piezoelectric resonator to become more narrow-banded. This has negative effects on the ways in which piezoelectric resonators may be utilized.
This reduction in the electromechanical coupling coefficients has two reasons. First, the electrical field forming in the amorphous silicon dioxide compensation layer results in a reduction of the electrical field in the piezoelectric layer 11 and thus in a deterioration of the electromechanical coupling.
The acoustic densification layers 9a, 9b, the piezoelectric layer 11 and the amorphous silicon dioxide layer are to be considered as a series connection of two resistors, wherein the voltage drop at the piezoelectric layer and the amorphous silicon dioxide layer depends on the conductivity of the respective layer. Since the acoustic densification layers 9a, 9b are characterized by a high electric conductivity, they do not influence the behavior of the series connection and the voltage drops at the piezoelectric layer 11 and the amorphous silicon dioxide layer.
In a piezoelectric resonator where only the piezoelectric layer 11 is arranged between the two electrodes 7a, 7b, the entire voltage drop would thus be at the piezoelectric layer 11, by which the electrical field forming therein would be greater than the electrical field forming in the piezoelectric resonator where the amorphous silicon dioxide layer is additionally arranged between one of the two electrodes 7a, 7b and the piezoelectric layer 11.
The amorphous silicon dioxide is characterized by a high resistivity, resulting in the amorphous silicon dioxide layer to be characterized by a poor electrical conductivity. This results in a considerable portion of the voltage between the two electrodes 7a, 7b to drop at the amorphous silicon dioxide layer in the series connection of the acoustic densification layers 9a, 9b, the amorphous silicon dioxide layer and the piezoelectric layer 11. This results in a decrease in the voltage drop at the piezoelectric layer 11 and thus in a reduction in the electrical field in the piezoelectric layer 11. This decrease in the electrical field in the piezoelectric layer 11 in turn causes a reduction of the electromechanical coupling coefficient of the piezoelectric resonator.
Apart from that, the amorphous silicon dioxide layer acting as the compensation layer for the temperature dependence of the resonance frequency comprises a relatively small acoustic resistance, which makes its usage in cooperating with the piezoelectric layer 11 and the electrode material in the piezoelectric resonator more difficult.
A reduction in the electromechanical coupling coefficient keff cannot be tolerated in many applications of the piezoelectric resonator when same is preferably embodied as a bulk acoustic wave resonator. This condition makes a usage of the amorphous silicon dioxide layer for compensating the temperature dependence of the resonance frequency in the piezoelectric resonator more difficult.
There is, for example, the requirement to bulk acoustic wave resonators in mobile telephones that the electromechanical coupling coefficient be above a critical value of 0.9.
DE 100 45 090 A1 shows an acoustic resonator having a first electrode and a second electrode and a piezoelectric resonator arranged therebetween. In the acoustic resonator, there is an acoustic densification layer between the piezoelectric layer and the first electrode, the acoustic densification layer comprising a higher acoustic impedance than the first electrode.
U.S. Pat. No. 4,456,850 shows a piezoelectric thin-film resonator where a thin film of silicon dioxide having a resonance frequency temperature coefficient of a sign opposite to a resonance frequency temperature coefficient of a piezoelectric material is inserted between two thin films of the piezoelectric material. This multi-layered setup is arranged between two electrode films and deposited onto a substrate.
In the documents Applied Physics Letters, vol. 74, no. 20 of May 17th, 1999, article “Eigenschaften von Aluminiumnitrid-Dünnfilmen für piezoelektrische Wandler und Mikrowellen-Filter-Anwendungen”, thin-film bulk acoustic resonators having a resonance frequency temperature coefficient of approximately zero are discussed. In the studies mentioned there, it is explained that the positive resonance frequency temperature coefficient of an SiO2 layer has a stabilizing effect on the resonance frequency since it compensates the negative resonance frequency temperature coefficient of a piezoelectric layer made of AlN.